the powery mildew look is a spray of sulfur i sprayed for that reason.
Do not know about the mildew effects but is there any chance you have a light leak? Erratic and leaking light can stress and confuse the plant try to send it back to veg, make it grow single leaves.
See no Evil, Speak no Evil, Smoke no Evil
Are they clones from a flowering plant?
Have a great one. Advice is for folks with a legal grow and worth what you paid for it.